Economic feasibility of methods for secondary and tertiary recovery of petroleum often depends on accurate measurement of the quantity and location of reservoir gas in a formation after previous recovery processes have been completed. Such measurements are desirably carried out in "old" wells, i.e., in wells used to produce the formation. Reasons: (i) accuracy is increased; and (ii) costs are decreased; the process of drilling a new well displaces some fraction of the formation fluids originally in the formation away from the hole, and it is desirable to evaluate the potential recovery from a reservoir without incurring the expenses of drilling a new well.
In my U.S. Pat. No. 3,817,328 for "Neutrons Absorption and Oxygen Log for Measuring the oil content of Formations," June 18, 1974, assigned to the assignee of this application, I describe a method for accurately determining the oil content of a reservoir containing both mobile oil and a significant gas saturation including recording the response of both thermal-neutron-decay-time and neutron-activated-oxygen logs. A purposeful change was then made in the fluids in a given region of the formation surrounding the well bore by injecting fluid under sufficient pressure to displace them. The combination of the thermal-neutron-decay-time log and the oxygen log was then run again, and differences between the two sets of logs noted.
My method may be somewhat limited, however, by the requirement that the oxygen activation log be calibrated at least to the extend that changes between the separate logs must be proportional to changes in the oxygen content of the reservoir fluids with a predetermined single constant of proportionality. Thus, my method could not be utilized for accurate gas saturation indications in some applications, say where the responses from logging tools currently available are influenced by the pipe, cement and liquids in the well bore; experience indicates that calibration valid at all depths in the well is difficult (if not impossible) to achieve in such situations.
I am also aware of the contributions of others in the oil-content, water- and gas-saturation measuring fields. Such contributions (and their limitations) are set forth in detail in my copending applications, Ser. Nos. 563,921 and 563,922, filed concurrently herewith, op.cit.; suffice it to say, with regard to gas-saturation measurements, none of the methods, as far as I am aware, have been quantitative enough to generate accurate direct determination of gas content of a reservoir or to provide indirect data ("inferential and results"), say from total hydrocarbon and oil saturation measurements from which such gas saturations can be accurately calculated.